The present invention relates to a magnetic hard disk drive which records and reproduces magnetic information by a perpendicular recording system using a recording medium including a soft magnetic layer.
The one, which has been widely investigated as an appropriate geometry for putting into practical use a magnetic hard disk using a perpendicular recording method, uses a magnetic inductive read head, wherein a single magnetic pole type write head is used as a write element, a multi-layer structured perpendicular recording medium including a soft magnetic layer (so-called soft magnetic lining layer) is used as a recording medium, and a magnetoresistive effect (MR effect) is used as a read element. However, heretofore, the generation of noise caused by magnetic domain walls formed inside the soft magnetic layer in the recording medium, so-called spike noise, has been confirmed (for instance, IEEE TRANSACTIONS ON MAGNETICS, VOL. 29, NO. 6, NOVEMBER 1993, pp. 3742-3744). Spike noise is a noise, which is superimposed on a read waveform when the read head passes over a magnetic domain wall (boundary of a magnetic domain) generated inside the soft magnetic layer of the recording medium, and has the following characteristics.    (1) The generation site is irregular (it is not always generated at a specific site in the recording medium).    (2) It moves (it is likely to move to other places when information is rewritten on the site where spike noise is generated).    (3) It becomes a long error (superimposition of the spike noise causes a base line shift and a distortion in the read waveform and introduces reading errors; the length reaches several ten to several hundred micrometers (corresponding to several hundred to several thousand bits) although it depends on the size of the magnetic domain wall).
Heretofore, as measures to counter such spike noise, various techniques for preventing the creation of noise-causing magnetic domain walls inside the soft magnetic layer while fabricating a recording medium have been examined and proposed (for instance, see JP-A No. 298326/2002, No. JP-A No. 168207/2003, and JP-A No. 45015/2003.
On the other hand, measures have been examined to counter the spike nose generated in spite of the above-mentioned countermeasures, such as using a TA (Thermal Asperity) compensation function in the reproducing signal processing LSI or registering it as a defect in the medium defect registration processing before product shipment. However, because the TA compensation function of a commercial LSI assumes that the base line shift of the length is only at most on the order of several tens of bits, there is difficulty dealing with the spike noise. In addition, even if medium defect registration processing is attempted, there is also difficulty because the location of the spike noise is irregular; therefore, a definitive solution has not been found yet. Moreover, in JP-A No. 100020/2002, a means is described in which a magnetic field is always applied to the recording medium using a permanent magnet and the entire soft magnetic layer or a wide area is uniformly magnetized to prevent generation of magnetic domain walls.